Improved support for Ubuntu Unity

Description

It would be nice if I could have individual virtual machines pinned to the unity taskbar. Adding the icon to the desktop doesn't quite achieve this functionality as if I remove it from the desktop, the Unity bar entry disappears.

Most ideal way would likely be to have each VM create a separate entry in the Unity bar, and allow them to be pinned from the right click menu.

Beyond that, perhaps the VirtualBox team can explore other things they may want to put in those menus. Probably some neat options!